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Abstract

We demonstrate a new class of soft matter optical fibers, which are self-assembled in a form of smectic-A liquid crystal microtubes grown in an aqueous surfactant dispersion of a smectic-A liquid crystal. The diameter of the fibers is highly uniform and the fibers are highly birefringent. They are characterized by a line topological defect in the core of the fiber with an optical axis pointing from the defect core towards the surface. We demonstrate guiding of light along the fiber and Whispering Gallery Mode (WGM) lasing in a plane perpendicular to the fiber. The light guiding as well as the lasing threshold are significantly dependent on the polarization of the excitation beam. The observed threshold for WGM lasing is very low (≈ 75μJ/cm2) when the pump beam polarization is perpendicular to the direction of the laser dye alignment and is similar to the lasing threshold in nematic droplets. The smectic-A fibers are soft and flexible and can be manipulated with laser tweezers demonstrating a promising approach for realization of soft photonic circuits.

Figures (5)

Fig. 1 Schematic diagram of the experimental set-up. The position of the focused infrared beam is controlled by two acousto-optic deflectors (AODs), thus allowing for the manipulation and tweezing of the smectic 8CB fibers in the aqueous C16TAB solution. The green pulsed beam from the doubled Nd:YAG laser is used to induce the fluorescence of the Nile red molecules which are dissolved and captured inside the 8CB fibers. The polarization of the incident beam can be varied with respect to the fiber axis. The continuous Ar+ laser is used to study the waveguiding properties of smectic fiber. The imaging spectrometer (Andor, Shamrock SR-500i), equipped with a cooled EM-CCD camera (Andor, Newton DU970N), is used to measure the optical spectrum emitted from the fiber at a 0.05nm resolution. The CCD camera is used to take photomicrographs of the samples.

Fig. 2 Polarizing microscopy and FCPM images of smectic-A 8CB microfibers (doped with 0.01wt.% of Nile red) in aqueous C16TAB solution. (a) 8CB microfiber (diameter ≈ 10μm) between crossed polarizers demonstrating the strong birefringence of the smectic fiber. The inset shows a thinner fiber (diameter ≈ 3μm), which has adopted a bend S-like shape, between crossed polarizers with the addition of a red (λ) wave plate. The alternating yellow/blue color sequence indicates that the LC molecules, and thus the local optical axis, are always aligned perpendicular to the surface of the fiber. (b) FCPM image of a smectic-A 8CB microfiber (diameter 35μm) with the polarization of the exciting light along the axis of the microfiber. High fluorescence intensity is observed at the spherical tip of the fiber. (c) FCPM image of the same fiber with the polarization of the exciting light perpendicular to the axis of the microfiber. High fluorescence intensity is observed along the cylindrical body of the fiber. The distribution of the fluorescence intensities in both images indicates that the Nile red molecules, and thus the LC molecules, are oriented perpendicular to the surface of the fiber. (d) Schematic drawings of the coaxial arrangement of the smectic layers in a microfiber. Top: cross section parallel to the fiber axis. Bottom: cross section perpendicular to the fiber axis. The red line or dot indicates the topological line defect. The rodlike LC molecules (not shown in the drawings) are oriented perpendicular to the layer planes and the fiber surface.

Fig. 3 Light guiding through 8CB smectic-A microfibers doped with 0.01 wt % of Nile red. The concentration of C16TAB surfactant in the surrounding aqueous medium is 20mM. The power of the applied continuous Ar+ laser beam for fluorescence excitation is 1mW. (a–b) A focused Ar+ beam is positioned at different points at the hemispherical cap of a 20μm thick fiber. The spiral-shaped trajectories of the guided light are clearly visible because of the fluorescence. (c–d) Light guiding by a thin (≈ 2μm) 8CB microfiber. The fluorescence is excited at the lower (c) or left (d) end of the microfiber using different polarizations of the Ar+ beam. The insets show the respective other end of the fiber at a higher magnification, the bright spot at the fiber end indicating the leaking of the guided light. The angle between the major axis of the microfiber and the polarization of the Ar+ beam is 0° in (c) and 90° in (d). The corrugated appearance of the thin fiber is a result of some random bending and the small thickness of the fiber. There is no indication for an axial variation of the fiber thickness. (e) The intensity I of emitted fluorescent light as a function of position (i.e. length) along the thin microfiber in (c,d) remains fairly constant. Note the intensity peak at the end of the microfiber, corresponding to the bright spot shown in the insets in (c,d).

Fig. 4 An example of lasing from a large, ≈ 50μm-diameter Nile red-doped 8CB microfiber in a 100mM C16TAB water solution. The power of the pumping laser is below the lasing threshold in (a) and above the threshold in (b). In both cases the pumping pulsed laser beam is illuminating a ≈ 20μm diameter region encircling the black cross. The polarization of the pumping light is along the tangent to the fiber at this position. Note the very distant laser speckles in (b), shining from the surface of the fiber at hundreds of μm separation (see enlarged section shown in (c)). Thinner fibers (thickness of a few μm) showed essentially the same behavior.

Fig. 5 The WGM lasing from the Nile red dye-doped smectic A microfiber of 8CB in 100mM C16TAB water solution. (a) The lasing spectrum just above the threshold for lasing. (b) The lasing spectrum well above the threshold. In both cases the polarization of the pumping pulsed laser is along the major axis of the microfiber. The cut-off at 650nm is due to the IR cut-off filter inserted in the dual beam laser tweezers. (c) Intensity of the emitted spectral line as a function of the laser excitation energy. The inset shows the region around the lasing threshold with a higher resolution. The lasing threshold for this geometry (polarization of exciting beam along fiber axis) is at ≈ 75μJ/cm2 and the laser line width at the threshold is ≈ 0.2nm.